1. Field of the Invention:
This invention relates to an electrode film for a foil type film capacitor. Capacitors of this type include a paper capacitor, metallized paper capacitor, plastic film capacitor. This invention is particularly effective when it is applied to a plastic film capacitor.
2. Description of the Prior Art:
As shown in FIG. 1, a plastic film capacitor is generally manufactured by the following. Two sheets of dielectric film 1, 2 and two sheets of electrode film 3, 4 are alternately layered and wound by a winding machine. Then, after the lead terminals (not shown) are respectively fixed to the electrode films 3, 4, this wound body is heated and baked. FIG. 2 is a view of a plastic film capacitor thus manufactured. In this capacitor, the lead terminals 5, 6 are led out in the same direction from the wound body 7.
The dielectric films 1, 2 are composed of a plastic film such as the polystylene, polyethylene-terephtalete or polypropylene, while the electrode films 3, 4 are composed of a metal film such as aluminium, copper or tin.
The dielectric films 1, 2 of the capacitor have a thickness in the range of several .mu.m to several tens of .mu.m depending on the rated voltage.
On the other hand, the thickness of the electrode films 3, 4 are not related to the static capacitance of the capacitor, and when the film is thick, only the volume of capacitor increases and it is very uneconomical from the point of view of material cost. Therefore, the electrode films 3, 4 must be as thin as possible and in general, have a thickness selected in the range of 6.5 .mu.m to several tens of .mu.m.
The static capacitance C, of the wound foil type film capacitor is given by the following equation. EQU C=2.epsilon.S/3.6.pi..multidot.d (pF)
where,
S: Overlapping area of two Electrode films (cm.sup.2) PA1 d: Dielectric film thickness (cm) PA1 .epsilon.: Dielectric constant of dielectric film
In general, a plastic film capacitor has a capacitance in the range of several tens pF to several tens of thousand pF. This capacitor has various advantages, such as a linear temperature characteristic, less dielectric absorption and dielectric strain, a small allowable difference of static capacitance, and is widely used in electronic devices.
However, a capacitor of this type also has disadvantages. For instance, a capacitor having a small capacitance has a large fluctuation in capacitance and high cost as compared with a capacitor having a large capacitance. This is because a very narrow width electrode film is required for manufacturing a plastic film capacitor of 100 pF or less; that is, a capacitor having a very small capacitance value ranging from several pF to several tens of pF. Namely, in the case of an ordinary electrode film, an ingot of for example, aluminium is stretched to form a metal film 8 having a large area as shown in FIG. 3A. Thereafter, a constant tension is applied to this metal film 8 and it is cut by a slitter while it is, under this tension and thereby, the electrode film 8a of a specified width is obtained as shown in FIG. 3B.
Conventionally, no matter how excellent the cutting ability of the slitter, the width of film is restricted because of the mechanical strength of film itself. Particularly, a metal such as aluminium is very fragile and malleable and therefore a film having a thickness of several tens of .mu.m or less is limited to a minimum film width of about 4 mm. In the case of using a metal film having a thickness of several tens of .mu.m or more, the cutting of film having a thickness of 4 mm or less is possible but, burs are generated during the cutting operation because the high mechanical strength of the film.
FIG. 3C is a sectional view of an enlarged section of the film 8a. As shown in the figure, burs 9 are easily generated at the time of cuting the ordinary film 8a into the specified width and the edge 10 is angled. Therefore, when a capacitor is manufactured using such a film 8a, burs 9 may pass through the dielectric film resulting in the short-circuit or the edge 10 may pass into the dielectric film making it thinner. As a result, the dielectric breakdown may often occur.
On the other hand, a winding machine is used for winding a plastic film capacitor and currently an automatic winder is almost always used. To this automatic winder, a static capacitance or number of windings are previously input. Thus, the plastic film and electrode film, layered alternately, are wound automatically until the static capacitance or the number of turns of winding reaches the specified preset value. In the case of manufacturing a capacitor having a small capacitance of under 100 pF, the required capacitance value can be obtained only by several turns of the films (about 2 to 3 turns in case the diameter is about 3 mm) when an ordinary electrode film having a width of 4 mm or more is used.
Therefore, the manufacturing yield of a capacitor having a small capacitance is extremely poor, and it has actually been impossible to manufacture a capacitance of under 100 pF using an automatic winder. With this background, in the case of manufacturing existing plastic film capacitor of 100 pF or less, a worker must wind the electrode film which has been cut in such a manner as to have a width of 4 mm or more and the specified length, by inserting it manually between the dielectric films using a manual winder. The many manufacturing steps explained above results in only expensive capacitors.